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This document describes how to use the HTTP client included in Twisted
Web. After reading it, you should be able to make HTTP and HTTPS
requests using Twisted Web. You will be able to specify the request
method, headers, and body and you will be able to retrieve the response
code, headers, and body.

A number of higher-level features are also explained, including proxying,
automatic content encoding negotiation, and cookie handling.

This document assumes that you are familiar with Deferreds and Failures , and producers and consumers .
It also assumes you are familiar with the basic concepts of HTTP, such
as requests and responses, methods, headers, and message bodies. The
HTTPS section of this document also assumes you are somewhat familiar with
SSL and have read about using SSL in Twisted .

The twisted.web.client.Agent class is the entry
point into the client API. Requests are issued using the request method, which
takes as parameters a request method, a request URI, the request headers,
and an object which can produce the request body (if there is to be one).
The agent is responsible for connection setup. Because of this, it
requires a reactor as an argument to its initializer. An example of
creating an agent and issuing a request using it might look like this:

As may be obvious, this issues a new GET request for /
to the web server on example.com . Agent is
responsible for resolving the hostname into an IP address and connecting
to it on port 80 (for HTTP URIs), port 443 (for HTTPS
URIs), or on the port number specified in the URI itself. It is also
responsible for cleaning up the connection afterwards. This code sends
a request which includes one custom header, User-Agent . The
last argument passed to Agent.request is None ,
though, so the request has no body.

Sending a request which does include a body requires passing an object
providing twisted.web.iweb.IBodyProducer
to Agent.request . This interface extends the more general
IPushProducer
by adding a new length attribute and adding several
constraints to the way the producer and consumer interact.

The length attribute must be a non-negative integer or the constant
twisted.web.iweb.UNKNOWN_LENGTH . If the length is known,
it will be used to specify the value for the
Content-Length header in the request. If the length is
unknown the attribute should be set to UNKNOWN_LENGTH .
Since more servers support Content-Length , if a length can be
provided it should be.

An additional method is required on IBodyProducer
implementations: startProducing . This method is used to
associate a consumer with the producer. It should return a
Deferred which fires when all data has been produced.

IBodyProducer implementations should never call the
consumer’s unregisterProducer method. Instead, when it
has produced all of the data it is going to produce, it should only
fire the Deferred returned by startProducing .

For additional details about the requirements of IBodyProducer implementations, see
the API documentation.

Here’s a simple IBodyProducer implementation which
writes an in-memory string to the consumer:

If you want to upload a file or you just have some data in a string, you
don’t have to copy StringProducer though. Instead, you can
use FileBodyProducer .
This IBodyProducer implementation works with any file-like
object (so use it with a StringIO if your upload data is
already in memory as a string); the idea is the same
as StringProducer from the previous example, but with a
little extra code to only send data as fast as the server will take it.

If the connection or the request take too much time, you can cancel the
Deferred returned by the Agent.request method.
This will abort the connection, and the Deferred will errback
with CancelledError .

So far, the examples have demonstrated how to issue a request. However,
they have ignored the response, except for showing that it is a
Deferred which seems to fire when the response has been
received. Next we’ll cover what that response is and how to interpret
it.

Agent.request , as with most Deferred -returning
APIs, can return a Deferred which fires with a
Failure . If the request fails somehow, this will be
reflected with a failure. This may be due to a problem looking up the
host IP address, or it may be because the HTTP server is not accepting
connections, or it may be because of a problem parsing the response, or
any other problem which arises which prevents the response from being
received. It does not include responses with an error status.

If the request succeeds, though, the Deferred will fire with
a Response . This
happens as soon as all the response headers have been received. It
happens before any of the response body, if there is one, is processed.
The Response object has several attributes giving the
response information: its code, version, phrase, and headers, as well as
the length of the body to expect. In addition to these, the
Response also contains a reference to the request that it is
a response to; one particularly useful attribute on the request is absoluteURI :
The absolute URI to which the request was made. The
Response object has a method which makes the response body
available: deliverBody . Using the
attributes of the response object and this method, here’s an example
which displays part of the response to a request:

The BeginningPrinter protocol in this example is passed to
Response.deliverBody and the response body is then delivered
to its dataReceived method as it arrives. When the body has
been completely delivered, the protocol’s connectionLost
method is called. It is important to inspect the Failure
passed to connectionLost . If the response body has been
completely received, the failure will wrap a twisted.web.client.ResponseDone exception. This
indicates that it is known that all data has been received. It
is also possible for the failure to wrap a twisted.web.http.PotentialDataLoss exception: this
indicates that the server framed the response such that there is no way
to know when the entire response body has been received. Only
HTTP/1.0 servers should behave this way. Finally, it is possible for
the exception to be of another type, indicating guaranteed data loss for
some reason (a lost connection, a memory error, etc).

Just as protocols associated with a TCP connection are given a transport,
so will be a protocol passed to deliverBody . Since it makes
no sense to write more data to the connection at this stage of the
request, though, the transport only provides IPushProducer . This allows the
protocol to control the flow of the response data: a call to the
transport’s pauseProducing method will pause delivery; a
later call to resumeProducing will resume it. If it is
decided that the rest of the response body is not desired,
stopProducing can be used to stop delivery permanently;
after this, the protocol’s connectionLost method will be
called.

An important thing to keep in mind is that the body will only be read
from the connection after Response.deliverBody is called.
This also means that the connection will remain open until this is done
(and the body read). So, in general, any response with a body
must have that body read using deliverBody . If the
application is not interested in the body, it should issue a
HEAD request or use a protocol which immediately calls
stopProducing on its transport.

If the body of the response isn’t going to be consumed incrementally, then readBody can be used to get the body as a byte-string.
This function returns a Deferred that fires with the body after the request has been completed; cancelling this Deferred will close the connection to the HTTP server immediately.

Everything you’ve read so far applies whether the scheme of the request
URI is HTTP or HTTPS . However, to control the SSL
negotiation performed when an HTTPS URI is requested, there’s
one extra object to pay attention to: the SSL context factory.

Agent ‘s constructor takes an optional second argument, a
context factory. This is an object like the context factory described
in Using SSL in Twisted but has
one small difference. The getContext method of this factory
accepts the address from the URL being requested. This allows it to
return a context object which verifies that the server’s certificate
matches the URL being requested.

Here’s an example which shows how to use Agent to request
an HTTPS URL with no certificate verification.

The important point to notice here is that getContext now
accepts two arguments, a hostname and a port number. These two arguments,
a str and an int , give the address to which a
connection is being established to request an HTTPS URL. Because an agent
might make multiple requests over a single connection,
getContext may not be called once for each request. A second
or later request for a URL with the same hostname as a previous request
may re-use an existing connection, and therefore will re-use the
previously returned context object.

HTTP persistent connections use the same TCP connection to send and
receive multiple HTTP requests/responses. This reduces latency and TCP
connection establishment overhead.

The constructor of twisted.web.client.Agent
takes an optional parameter pool, which should be an instance
of HTTPConnectionPool , which will be used
to manage the connections. If the pool is created with the
parameter persistent set to True (the
default), it will not close connections when the request is done, and
instead hold them in its cache to be re-used.

Here’s an example which sends requests over a persistent connection:

fromtwisted.internetimportreactorfromtwisted.internet.deferimportDeferred,DeferredListfromtwisted.internet.protocolimportProtocolfromtwisted.web.clientimportAgent,HTTPConnectionPoolclassIgnoreBody(Protocol):def__init__(self,deferred):self.deferred=deferreddefdataReceived(self,bytes):passdefconnectionLost(self,reason):self.deferred.callback(None)defcbRequest(response):print'Response code:',response.codefinished=Deferred()response.deliverBody(IgnoreBody(finished))returnfinishedpool=HTTPConnectionPool(reactor)agent=Agent(reactor,pool=pool)defrequestGet(url):d=agent.request('GET',url)d.addCallback(cbRequest)returnd# Two requests to the same host:d=requestGet('http://localhost:8080/foo').addCallback(lambdaign:requestGet("http://localhost:8080/bar"))defcbShutdown(ignored):reactor.stop()d.addCallback(cbShutdown)reactor.run()

Here, the two requests are to the same host, one after the each
other. In most cases, the same connection will be used for the second
request, instead of two different connections when using a
non-persistent pool.

twisted.web.client.HTTPConnectionPool instances
have an attribute
called maxPersistentPerHost which limits the
number of cached persistent connections to the same server. The default
value is 2. This is effective only when the persistent option is
True. You can change the value like bellow:

With the default value of 2, the pool keeps around two connections to
the same host at most. Eventually the cached persistent connections will
be closed, by default after 240 seconds; you can change this timeout
value with the cachedConnectionTimeout
attribute of the pool. To force all connections to close use
the closeCachedConnections
method.

If a request fails without getting a response, and the request is
something that hopefully can be retried without having any side-effects
(e.g. a request with method GET), it will be retried automatically when
sending a request over a previously-cached persistent connection. You can
disable this behavior by setting retryAutomatically
to False . Note that each request will only be retried
once.

By itself, Agent doesn’t follow HTTP redirects (responses
with 301, 302, 303, 307 status codes and a location header
field). You need to use the twisted.web.client.RedirectAgent class to do so. It
implements a rather strict behavior of the RFC, meaning it will redirect
301 and 302 as 307, only on GET and HEAD
requests.

In contrast, twisted.web.client.BrowserLikeRedirectAgent implements
more lenient behaviour that closely emulates what web browsers do; in
other words 301 and 302 POST redirects are treated like 303,
meaning the method is changed to GET before making the redirect
request.

As mentioned previously, Response contains a reference to both
the request that it is a response
to, and the previously received response , accessible by previousResponse .
In most cases there will not be a previous response, but in the case of
RedirectAgent the response history can be obtained by
following the previous responses from response to response.

To be able to use HTTP proxies with an agent, you can use the twisted.web.client.ProxyAgent class.
It supports the same interface as Agent, but takes the endpoint of the proxy as initializer argument.
This is specifically intended for talking to servers that implement the proxying variation of the HTTP protocol; for other types of proxies you will want Agent.usingEndpointFactory (see documentation below).

Here’s an example demonstrating the use of an HTTP proxy running on
localhost:8000.

An existing agent instance can be wrapped with
twisted.web.client.CookieAgent to automatically
store, send and track HTTP cookies. A CookieJar
instance, from the Python standard library module
cookielib , is
used to store the cookie information. An example of using
CookieAgent to perform a request and display the collected
cookies might look like this:

twisted.web.client.ContentDecoderAgent adds
support for sending Accept-Encoding request headers and
interpreting Content-Encoding response headers. These headers
allow the server to encode the response body somehow, typically with some
compression scheme to save on transfer
costs. ContentDecoderAgent provides this functionality as a
wrapper around an existing agent instance. Together with one or more
decoder objects (such as
twisted.web.client.GzipDecoder ), this wrapper
automatically negotiates an encoding to use and decodes the response body
accordingly. To application code using such an agent, there is no visible
difference in the data delivered.

Implementing support for new content encodings is as simple as writing a
new class like GzipDecoder that can decode a response using
the new encoding. As there are not many content encodings in widespread
use, gzip is the only encoding supported by Twisted itself.

Typically you want your HTTP client to open a TCP connection directly to the web server.
Sometimes however it’s useful to be able to connect some other way, e.g. making an HTTP request over a SOCKS proxy connection or connecting to a server listening on a UNIX socket.
For this reason, there is an alternate constructor called Agent.usingEndpointFactory that takes an endpointFactory argument.
This argument must provide the twisted.web.iweb.IAgentEndpointFactory interface.
Note that when talking to a HTTP proxy, i.e. a server that implements the proxying-specific variant of HTTP you should use ProxyAgent - see documentation above.